This invention relates to a novel method for preparing phosphor that is better adapted to be deposited subsequently in dry photosensitive layers from an aqueous acid slurry, and to the product of that method. The product may be used, for example, to prepare a luminescent viewing screen for a cathode-ray tube by the slurry-direct photographic process.
The slurry-direct photographic process is described in U.S. Pat. Nos. 3,406,068 to H. B. Law, 3,313,643 to P. B. Branin, and 3,269,838 to T. A. Saulnier, for example. In the common practice of that process, an aqueous slurry is prepared containing the desired particulate phosphor; a water-soluble, photosensitizable binder for the phosphor, such as polyvinyl alcohol; and a water-soluble dichromate photosensitizer for the binder. The slurry is coated on the inner surface of the viewing window of the tube and then dried, producing the photosensitive layer. The dry photosensitive layer is exposed to a pattern of actinic radiation to insolubilize or harden selected areas of the layer. The exposed layer is then flushed with water to remove the unhardened areas of the layer, while retaining the insoluble, hardened areas of the layer in place, thereby producing the desired screen structure.
Acid slurries of some phosphors, particularly phosphors containing yttrium, lanthanum and/or gadolinium cations in the host crystal thereof, such as europium-activated yttrium oxysulfide, may form dry photosensitive layers which are said to be supersensitive. Supersensitive layers require far less exposure time to achieve a desired amount of hardening than layers that are not supersensitive. The supersensitivity is not consistent or predictable, and is frequently erratic, differing in some uncontrolled way from batch to batch of the same slurry composition. It is believed that, when yttrium oxysulfide phosphor is mixed in different batches of aqueous slurry, different concentrations of free yttrium cations are produced in the slurries. These cations are either leached from the surface of the phosphor or are formed by a minimal dissolution of the phosphor. Concentrations of about 100 ppm (parts per million) and higher of yttrium cations can produce dry layers that are unpredictably supersensitive, and thereby make it impossible to produce the desired screen structure in a precise and controlled manner. In general, slurries with free yttrium cation concentrations of less than 100 ppm will produce acceptable dry photosensitive layers. However, slurries with free cation concentrations that differ substantially from one slurry batch to another can produce erratic and unpredictable results when photoexposing layers are made from these slurries.
It has been suggested that supersensitivity occurs when a substantial amount of dichromate photosensitizer converts to chromate in the slurry and/or the layer, which causes the photosensitive layer to slowly harden without actinic radiation prior to being photoexposed. This effect, which is sometimes called "dark hardening," lowers the amount of light needed to achieve the desired amount of hardening by photoexposure in a manner analogous to a flood exposure. Conversion of dichromate to chromate in conjunction with dark hardening has been observed with dry photosensitive layers made from alkaline phosphor slurries (pH above 7.0).
In acid phosphor slurries (pH below 7.0), dichromate does not convert appreciably to chromate. However, the acidity of the slurry does, to a larger extent than in alkaline slurries, attack the phosphor particles in the slurry. It is believed that, in dry photosensitive layers of phosphors containing yttrium, lanthanum and/or gadolinium, such as europium-activated yttrium oxysulfide, and produced from an acid slurry, supersensitivity is accelerated by the presence therein of water-soluble salts of the same cations as are present in the cation portion of the phosphor. In the case of activated yttrium oxysulfide or vanadate phosphors, for example, salts of yttrium are present which accelerate dark hardening. The principal source of these salts is believed to be the phosphor itself which may have been incompletely washed after it was synthesized or salvaged, or which may have dissolved in the acid slurry.
In order to reduce the supersensitivity and dark hardening of photosensitive phosphor layers made from an aqueous slurry, it has been suggested to add to the slurry a soluble substance that combines with the cations of the soluble salts described above and renders these cations inactive in the dry photosensitive layer that is subsequently formed from that slurry. The soluble substance may be a chelating agent or a complexing agent, for example. Alternatively, the phosphor particles may be washed with a solution of the soluble substance before incorporation into the slurry. It has also been suggested to coat or encapsulate the phosphor particles with a substance that protects the particles from dissolution in the slurry. The prior suggestions have had only partial success at best in acid slurries and are considered to be inadequate for accurate process control in modern automated manufacturing facilities.